1. Field of the Invention
The invention relates to an exhaust gas purification device for an engine.
2. Description of the Related Art
An exhaust gas purification device for an engine is known. The device comprises a catalyst arranged in an exhaust passage for purifying NO.sub.x in an exhaust gas. The catalyst reduces the NO.sub.x with HC (hydrocarbon) to N.sub.2 to purify the NO.sub.x at a ratio higher than a predetermined ratio when a temperature of the catalyst is within a predetermined temperature range. Therefore, it is preferable to maintain the temperature of the catalyst within the predetermined temperature range in order to purify the NO.sub.x at high ratio.
In an exhaust gas purification device for an engine disclosed in Japanese Patent Publication No. 63-283727, HC is fed into an exhaust gas when an amount of HC is not sufficient to purify NO.sub.x.
The NO.sub.x in the exhaust gas includes NO and NO.sub.2. Generally, the HC activated in the catalyst can easily reduce the NO.sub.2 to N.sub.2 while the activated HC cannot easily reduce the NO to N.sub.2. In order to purify the NO, it is necessary to oxidize NO with O.sub.2 in the exhaust gas to NO.sub.2, and thereafter, the NO.sub.2 is reduced with the HC to the N.sub.2.
However, the NO cannot easily be oxidized with O.sub.2 to NO.sub.2 when the temperature of the catalyst is within the predetermined temperature range since, in the catalyst, a reducing reaction is easily carried out.